Thin film transistor substrate and liquid crystal display unit having a low-resistance silicon compound film

ABSTRACT

The present invention provides a thin-film transistor substrate comprising: a gate electrode  40  and a gate insulating film  41  formed on a substrate  36;  a semiconductor active film  42  oppositely provided on the gate electrode via the gate insulating film; a pair of ohmic contact films  43  and  44  separately provided on the semiconductor active film; a low-resistance silicon compound film  45  ranging from the ohmic contact films to the gate insulating film so as to cover the ohmic contact films and the portions of the semiconductor active film superposing with the ohmic contact films; and a source electrode  46  and a drain electrode  48  provided on the low-resistance silicon compound film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin-film transistor substrate, aliquid crystal display unit provided with the same and a manufacturingmethod of the thin-film transistor substrate. More particularly, theinvention relates to a technique capable of improving transistorproperties by using a special structure for the connecting portionbetween source and drain electrodes and a semiconductor active film.

2. Description of the Related Art

FIGS. 6 and 7 illustrate a typical structure of a thin-film transistorarray substrate provided with various parts such as a gate wiring G anda source wiring S on a substrate in a conventional thin-film transistortype liquid crystal display unit. In the thin-film transistor arraysubstrate shown in FIGS. 6 and 7, the gate wiring G and the sourcewiring S are arranged in a matrix shape on a transparent substrate 6such as a glass one. The region surrounded by the gate wiring G and thesource wiring S serves as a picture element section 1 in which athin-film transistor 3 is provided.

The thin-film transistor 3 shown in FIGS. 6 and 7 has a generalconfiguration of the etch-stopper type, and comprises a gate insulatingfilm 9 provided on a gate wiring G and an gate electrode 8 provided byextracting from the gate wiring G; a semiconductor active film 10comprising amorphous silicon (a-Si) provided on this gate insulatingfilm 9 oppositely to the gate electrode 8; and a drain electrode 11 anda source electrode 12 comprising a conductive material provided oppositeto each other on this semiconductor active film 10. On the upper sidesof the both sides of the semiconductor active film 10, there are formedohmic contact films 10 a and 10 a made of amorphous silicon by dopingimpurities serving as donors such as phosphorus at high concentrations,and an etching stopper 13 held between the drain electrode 11 and thesource electrode 12 is formed thereon. A transparent picture elementelectrode 15 comprising a transparent electrode material is connected soas to range from above the drain electrode 11 to a side of the drainelectrode 11.

A passivation film 16 is provided so as to cover the gate insulatingfilm 9, the transparent picture element electrode 15, the drainelectrode 11 and the source electrode 12. An orientation film not shownis formed on the passivation film 16. An active matrix liquid crystaldisplay unit is formed by providing a liquid crystal above thisorientation film, so that impression of an electric field to liquidcrystal molecules by means of the transparent picture element electrode15 permits orientation control of the liquid crystal molecules.

The liquid crystal display unit of the above-mentioned construction hasa configuration in which a back light is provided on the back of thetransparent substrate 6, and the user can recognize bright or dark fromwhether the orientation-controlled liquid crystal interrupts or allowsto transmit a light emitted from the back light.

However, when a part of the light entering the transparent substrate 6should reach the semiconductor active film 10 between the drainelectrode 11 and the source electrode 12, a charge is produced in thesemiconductor active film 10 through excitation by the light, causingphotocurrent to flow. When driving the thin-film transistor, therefore,leakage current would flow although the circuit is turned off. The flowof such leakage current causes an increase in turnoff current (I_(OFF))during driving of the liquid crystal, and this may adversely affectlight transmissivity of the liquid crystal.

For the purpose of avoiding this inconvenience, there is proposed astructure in which the gate electrode 8 is formed of a light shieldingconductive film by forming the gate electrode 8 into a size larger thanthe semiconductor active film 10 so as to prevent the light of the backlight from reaching the semiconductor active film 10.

FIG. 8 illustrates a typical thin-film transistor structure of thiskind: the thin-film transistor 27 comprises a gate electrode 21 made ofa light-shielding conductive material provided on a substrate 20; a gateinsulating film 22 covering the gate electrode 21; a semiconductoractive film 23 smaller than the gate electrode 21 provided opposite tothe gate electrode 21 on the gate insulating film 22; ohmic contactfilms 24 and 24 provided on the both sides of the semiconductor activefilm 23; a source electrode 25 covering one of the ohmic contact films24; and a drain electrode 26 covering the other ohmic contact film 24.

With the structure shown in FIG. 8, in which the gate electrode 21serves also as a light shielding layer, it is possible to prevent thelight from the back light from entering the semiconductor active film23, and ensure satisfactory electrical contact of the source electrode25 and the drain electrode 26 with the semiconductor active film 23under the effect of the ohmic contact films 24 and 24.

With the structure shown in FIG. 8, however, measurement of OFF-current(I_(OFF)) and ON-current (I_(ON)) as a thin-film transistor gives acurve as shown in FIG. 10, suggesting a problem of impossibility toachieve a sufficiently low OFF-current. As a result of search for causescarried out by the present inventors, this is attributable to the factthat, in the structure shown in FIG. 8, an end of the semiconductoractive film 23 imparted a strong electric field facing the sourceelectrode 25 is in direct contact with the source electrode 25, or anend of the semiconductor active film 23 facing the drain electrode 26 isin direct contact with the drain electrode 26, at portion e in FIG. 8,and this makes it impossible to obtain a sufficient Hall blockingeffect.

FIG. 9 illustrates another example of conventional structure ofthin-film transistor. The thin-film transistor 28 in this examplecomprises an ohmic contact film 29 provided so as to cover the end ofthe semiconductor active film 23 and the gate insulating film 22 on theside thereof, i.e., so as to be laminated over the source electrode 25at the bottom of the source electrode 25, and another ohmic contact film29 provided so as to be laminated over the drain electrode 26 at thebottom of the drain electrode 26.

In the structure shown in FIG. 9, however, measurement of OFF-current(I_(OFF)) and ON-current (I_(ON)) as a thin-film transistor results in acurve b as shown in FIG. 10: while the value of OFF-current can bereduced sufficiently, the value of ON-current cannot be increased.

When manufacturing the thin-film transistor 28 of the structure shown inFIG. 9, the semiconductor active film 23 is once formed on the entireupper surface of the gate insulating film 22 to form the semiconductoractive film 23 on the gate insulating film 22. When patterning this filmto achieve the semiconductor active film 23 having an island shape witha target size, the upper surface of the semiconductor active film 23 issusceptible to easy contamination, and even formation of the ohmiccontact films 29 thereafter cannot ensure sufficient electric connectionbetween the semiconductor active film 23 and the ohmic contact films 29.This is considered to be a cause of the above inconvenience.

SUMMARY OF THE INVENTION

The present invention was developed in view of the above-mentionedcircumstances, and has an object to provide a thin-film transistor and aliquid crystal display unit provided therewith in which a sourceelectrode and a drain electrode are connected to a semiconductor activefilm so as not to cause mutual contact, via a low-resistance siliconcompound film, ON-current being increased by improving electricconnection of these components, and OFF-current is reduced by improvingelectric connection between the semiconductor active film and ohmiccontact films. Another object of the invention is to provide amanufacturing method of a thin-film transistor having such a structure.

To solve the above-described objects, the present invention provides athin-film transistor substrate comprising a gate electrode provided on asubstrate; a gate insulating film provided on the substrate so as tocover the gate electrode; a semiconductor active film providedoppositely via the gate insulating film above the gate electrode; a pairof ohmic contact films separately provided on the semiconductor activefilm; a low-resistance silicon compound film ranging from the ohmiccontact film to the gate insulating film so as to cover the respectiveohmic contact films and the portion of the semiconductor active filmsuperposed on the ohmic contact films; and a source electrode and adrain electrode provided on the low-resistance silicon compound film.

By adopting the configuration as described above, the source and drainelectrodes never come into direct contact in part with the semiconductoractive film, and connection of the source and drain electrodes to thesemiconductor active film is accomplished via the ohmic contact filmsand the low-resistance silicon compound film, thus permitting reductionof OFF-current.

Lamination of the ohmic contact films only on the upper surface of thesemiconductor active film permits fabrication by patterning of thesemiconductor active film and the ohmic contact films into a necessaryshape after lamination of the both films. It is thus possible to performsufficient electric connection between the lamination-formedsemiconductor active film and the ohmic contact films and to give asufficiently high ON-current.

To solve the above-mentioned problems, in the liquid crystal displayunit of the invention, the thin-film transistor substrate serves as oneof a pair of substrates having a liquid crystal layer in between.

By using such a configuration, it is possible to provide a liquidcrystal display unit having a thin-film transistor having a smallOFF-current and a large ON-current.

The invention provides a manufacturing method of a thin-film transistorsubstrate, comprising the steps of: forming a gate electrode on asubstrate; forming sequentially a gate insulating film covering the gateelectrode, a semiconductor film, and an impurity semiconductor filmadded with impurities; etch-forming the semiconductor film and theimpurity semiconductor film into a semiconductor active film and animpurity semiconductor film of desired shapes above the gate electrodeoppositely thereto; sequentially and continuously forming alow-resistance silicon compound film and an electrode film so as tocover the semiconductor active film, the impurity semiconductor film andthe gate insulating film; and etching the impurity semiconductor film,the low-resistance silicon compound film and the electrode film, therebyforming a pair of isolated ohmic contact films, laminated low-resistancesilicon compound film and source electrode ranging from the individualohmic contact films to the gate insulating film, and laminatedlow-resistance silicon compound film and drain electrode.

By manufacturing a thin-film transistor by the above-mentioned method,the source and the drain electrodes are never brought into directcontact with the semiconductor active film, but are connected to thesemiconductor active film via the ohmic contact films and thelow-resistance silicon compound film. There is therefore available athin-film transistor capable of reducing OFF-current.

Lamination of the ohmic contact films only on the upper surface side ofthe semiconductor active film permits fabrication by patterning of thesemiconductor active film and the ohmic contact films into a necessaryshape after lamination of the both films. It is thus possible to performsufficient electric connection between the lamination-formedsemiconductor active film and ohmic contact films and to give asufficiently high ON-current.

The present invention also provides a manufacturing method of thin-filmtransistor substrate, comprising the steps of: forming a gate electrodeon a substrate; forming sequentially a gate insulating film covering thegate electrode, a semiconductor film, and an impurity semiconductor filmadded with impurities; etch-forming the semiconductor film and theimpurity semiconductor film into a semiconductor active film and animpurity semiconductor film of desired shapes above the gate electrodeoppositely thereto; forming a metal film so as to cover thesemiconductor active film, the impurity semiconductor film, and the gateinsulating film and simultaneously forming a low-resistance siliconcompound film over the portion of the metal film in contact with thesemiconductor active film and the impurity semiconductor film;etch-removing the metal film to leave only the low-resistance siliconcompound film; forming an electrode film so as to cover thesemiconductor active film, the impurity semiconductor film, and theremaining low-resistance silicon compound film; etching the impuritysemiconductor film, the low-resistance silicon compound film, and theelectrode film to form a pair of isolated ohmic contact films, a sourceelectrode and a drain electrode ranging from the individual ohmiccontact films to the gate insulating film.

By continuously forming the semiconductor active film and the ohmiccontact films on the gate insulating film, these films can be laminatedwith a cleaned interface therebetween, thus making it possible toachieve satisfactory electric connection between the both films.

Through mutual dispersion of elements in a heat treatment applied to thelow-resistance silicon compound preparatory film covering thesemiconductor active film and the ohmic contact films, it is possible toform a low-resistance silicon compound film covering the semiconductoractive film and the ohmic contact films. By forming a source electrodeand a drain electrode on this low-resistance silicon compound film, itis possible to form the source electrode and the drain electrode whichare connected to the semiconductor active film via the low-resistancesilicon compound film without coming into direct contact with thesemiconductor active film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a liquid crystal display unit and athin-film transistor in a first embodiment of the present invention;

FIG. 2 illustrates a manufacturing method of the thin-film transistorshown in FIG. 1: FIG. 2(A) is a sectional view illustrating a gateelectrode, a gate insulating film, a semiconductor active film and ohmiccontact films formed on a substrate; FIG. 2(B) is a sectional viewillustrating a low-resistance silicon compound film and a conductivefilm formed on the ohmic contact films; and FIG. 2(C) is a sectionalview of the thin-film transistor;

FIG. 3 is a sectional view of a liquid crystal display unit and athin-film transistor in a second embodiment of the invention;

FIG. 4 illustrates a manufacturing method of the thin-film transistorshown in FIG. 3: FIG. 4(A) is a sectional view illustrating a gateelectrode, a gate insulating film, a semiconductor active film and ohmiccontact films formed on a substrate; FIG. 4(B) is a sectional viewillustrating a silicide preparatory thin film formed on the ohmiccontact films; FIG. 4(C) is a sectional view illustrating alow-resistance silicon compound film formed by a heat treatment; andFIG. 4(D) is a sectional view of the thin-film transistor;

FIG. 5 is a graph illustrating properties of ON-current and OFF-currentof the thin-film transistor obtained in examples;

FIG. 6 is a schematic plan view of a picture element section in anexample of the conventional liquid crystal display unit;

FIG. 7 is a sectional view of the liquid crystal display unit shown inFIG. 6;

FIG. 8 is a sectional view illustrating an example of the conventionalthin-film transistor;

FIG. 9 is a sectional view illustrating another example of theconventional thin-film transistor; and

FIG. 10 is a graph illustrating characteristics of ON-current andOFF-current of the typical conventional thin-film transistors.

DESCRIPTION OF THE EMBODIMENTS

A few embodiments of the invention will now be described in detail. Theinvention is not limited in any manner by these embodiments.

FIG. 1 illustrates portions of a first embodiment of a liquid crystaldisplay unit of the invention. The liquid crystal display unit 30 ofthis embodiment comprises a thin-film transistor array substrate 31, atransparent opposed substrate 32 provided separately in parallel withthe thin-film transistor array substrate 31, and a liquid crystal layer33 sealed between the thin-film transistor array substrate 31 and theopposed substrate 32.

In this thin-film transistor array substrate 31, a plurality of sourcewiring lines in a longitudinal rows and a plurality of gate wiring linesin a transverse rows are arranged so as to form a matrix shape as viewedflat from the upper surface side of the opposed substrate 32. Aplurality of regions surrounded by the source wiring lines and the gatewiring lines are called picture element sections. A picture elementelectrode 35 made of a transparent conductive material such as ITO(indium-tin oxide) is formed in each of the regions corresponding to theindividual picture element sections, and a thin-film transistor T isprovided near each picture element electrode 35.

FIG. 1 is a sectional view illustrating, in an enlarged form, theportion of the thin-film transistor T provided in each of the regionscorresponding to a picture element section surrounded by the sourcewiring lines and the gate wiring lines and the proximities thereof. Aplurality of picture element sections are formed in rows on thethin-film transistor array substrate 31 to form a display screen of aliquid crystal display unit 30.

The thin-film transistor array substrate 31 in this form comprises agate electrode 40 made of a light-shielding conductive material such asCr or Mo provided on a substrate 36 in each picture element section; agate insulating film 41 so as to cover the gate electrode 40 and thesubstrate 36; a semiconductor active film 42 smaller in size than thegate electrode 40 laminated on the gate insulating film 41 on the gateelectrode 40; and ohmic contact films 43 and 44 laminated separatelyfrom each other with a gap on the side of the center portion of thesemiconductor active film 42, aligned with the ends of the semiconductoractive film 42. The gate electrode 40 is formed into a size large inwidth as well as in depth than the semiconductor active film 42 so as toprevent the light from a back light provided under the substrate 36 fromentering the semiconductor active film 42.

Then, a low-resistance silicon compound film 45 comprising an a-Si:n⁺layer or chromium silicide is provided so as to cover the upper surfaceand the left side surface of the ohmic contact film 43 to the left inFIG. 1 (far side from the picture element electrode 35 in FIG. 1), theleft side surface of the semiconductor active film 42 locatedthereunder, and part of the upper surface of the gate insulating film 41continuing thereto, i.e., to cover the superposing portion of thesemiconductor active film 42 and the ohmic contact film 43. Thereupon, asource electrode 46 comprising a highly conductive metal material suchas Al or an Al alloy is formed. A low-resistance silicon compound film47 comprising an n⁺ layer or the like is formed so as to cover the uppersurface and the right side surface of the ohmic contact film 43 to theright in FIG. 1 (closer side to the picture element electrode 35 in FIG.1), the right side surface of the semiconductor active film 42 locatedthereunder, and part of the upper surface of the gate insulating film 41continuing thereto, i.e., to cover the superposing portion of thesemiconductor active film 42 and the ohmic contact film 43. Thereupon, adrain electrode 48 comprising a highly conductive metal material such asAl is formed.

A passivation film 49 is provided on these films so as to cover thesame. A picture element electrode 35 is formed on the portion of thepassivation film 49 on the right end of the drain electrode 48. Thispicture element electrode 35 is connected to the drain electrode 48 viaa connecting conductor 51 provided in a contact hole 90 formed in thepassivation film 49.

On the liquid crystal side of the opposed substrate 32 provided relativeto the thin-film transistor array substrate 31, there are laminated,sequentially from the side near the opposed substrate 32, a color filter52 and a common electrode film 53. The color filter 52 mainly comprisesa black matrix 54 for covering portions not contributing to display,such as part of the thin-film transistor, part of the gate wiring andpart of the source wiring, and color picture element sections 55 forallowing a light to transmit the portion of the picture element regionhaving the picture element electrode 35 contributing to display, andalso for conducting color display. These color picture element sections55 are required for a liquid crystal display unit has a structure forcolor display, and provided one for each picture element section. R(red), G (green) and B (blue) color picture element sectionsrepresenting three original colors are arranged regularly or at randomso as to avoid deviation of colors so that adjacent picture elementsections are different in color from each other.

In the sectional structure shown in FIG. 1, orientation films providedon the liquid crystal side of the thin-film transistor array substrate31 and on the liquid crystal side of the opposed substrate 32 areomitted, and polarizing plates provided on the outside of the thin-filmtransistor array substrate 31 and on the outside of the opposedsubstrate 32.

In the structure shown in FIG. 1, the source electrode 46 and the drainelectrode 48 are connected to the semiconductor active film 42 via thelow-resistance silicon compound films 45 and 47, and further, via theohmic contact films 43 and 44 on the semiconductor active film, and thesource electrode 46 and the drain electrode 48 are not in direct contactwith the semiconductor active film 42. Hall blocking effect is thereforecertainly available. Furthermore, since there is no risk of the highlyconductive metal material such as Al or an Al alloy composing the sourceelectrode 46 and the drain electrode 48 being dispersed in thesemiconductor active film, it is possible to reduce OFF-current as athin-film transistor T.

When laminating the ohmic contact films 43 and 44 on the semiconductoractive film 42, if in the structure shown in FIG. 1, it is possible tocontinuously form films without the need to take out the substrate fromthe vacuum film forming apparatus. It is therefore possible to achievesatisfactory electrical connection on the interface between thesemiconductor active film 42 and the ohmic contact films 43 and 44. Itis thus possible to obtain a satisfactory value of ON-current for athin-film transistor of this embodiment.

A typical manufacturing method of the thin-film transistor T having thestructure shown in FIG. 1 will now be described with reference to FIG.2.

The process comprises the steps of first preparing a substrate 36 shownin FIG. 2(A); forming a conductive film made of a light-shieldingconductive metal material such as Cr or Mo by means of a film formingapparatus such as a sputtering unit on this substrate; then, coatingthereon a resist for pattern exposure; removing unnecessary portionsthrough etching; then, applying patterning for peeling off the resist;forming a plurality of gate wiring lines (not shown) on the substrate,and at the same time, forming a gate electrode 40 connected to the gatewiring for each position corresponding to the picture element section.

The process comprises further steps of, upon forming the gate electrode40, forming a gate insulating film 41 of an insulating material such asSiN_(x) so as to cover the gate wiring and the gate electrode 40 bymeans of a film forming apparatus; and then, continuously forming anisland-shaped semiconductor active film 42 smaller in size than the gateelectrode 40 and an ohmic contact film 43 a at a position opposite tothe gate electrode 40 at the gate insulating film 41 on the gateelectrode 40. In order to form the semiconductor active film 42 and theohmic contact film 43 a, it suffices to continuously laminating filmsserving as materials for these films over the entire surface of the gateinsulating film 41 by means of a film forming apparatus, and then, usingportions remaining after removal of unnecessary portions by etching asthe semiconductor active film 42 and the ohmic contact film 43 a.

A low-resistance silicon compound film 50 such as an n⁺ layer formed byadding elementary phosphorus to a semiconductor material is formed so asto cover the ohmic contact film 43 a and the gate insulating film 41 asshown in FIG. 2(B), followed by formation of a conductive film 51comprising a conductive metal material such as Al or an Al alloy onthese films. If the low-resistance silicon compound film 50 and theconductive film 51 are continuously formed in a vacuum atmosphere in afilm forming apparatus, the low-resistance silicon compound film 50 andthe conductive film 51 can be satisfactorily connected and formedwithout allowing impurities to be present in the interface between thesefilms.

Then, ohmic contact films 43 and 44 are formed separately from eachother on the both ends of the semiconductor active film 42 by removingby etching the upper portion of the center portion of the semiconductoractive film 42, and removing portions of the ohmic contact film 43 a,the low-resistance silicon compound film 50 and the conductive film 51on the center portion of the semiconductor active film 42 as shown inFIG. 2(C). It is thus possible to form the low-resistance siliconcompound films 45 and 47, the source electrode 46 and the drainelectrode 48 in the covered form on the individual ohmic contact film,and also to obtain a thin-film transistor T having a structure in whichthe superposed portion of the semiconductor active film 42 and the ohmiccontact films 43 and 44 are covered by the low-resistance siliconcompound films 45 and 47, as shown in FIG. 2(C).

Although not shown in the drawing, the source wiring may besimultaneously formed upon forming the film or during etching whenforming the source electrode 46 on the gate insulating film 41.

When manufacturing a thin-film transistor T by the method describedabove, it is possible to continuously form the semiconductor active film42 and the ohmic contact film 43 a formed thereon, and also tocontinuously laminate the both layers in an identical vacuum atmospherewithout breaking the vacuum atmosphere in the film forming apparatus. Itis thus possible to connect and form in a satisfactory state ofelectrical connection the semiconductor active film 42 and the ohmiccontact film 43 a formed thereon without causing impurities to be mixedin the connection interface between the both films. It is also possibleto improve the state of electrical connection at the connectioninterface between the low-resistance silicon compound film 45 and thesource electrode 46, and the state of electrical connection at theconnection interface between the low-resistance silicon compound film 47and the drain electrode 48. In the structure shown in FIG. 2(C), thesource electrode 46 has no part in direct contact with the semiconductoractive film 42, and the drain electrode 48 has no part in direct contactwith the semiconductor active film 42. It is therefore possible toreduce OFF-current as a thin-film transistor T.

As a result, there is available a thin-film transistor substrate havinga thin-film transistor T excellent in transistor characteristics with ahigh ON-current and a low OFF-current.

FIG. 3 illustrates important portions of a second embodiment of theliquid crystal display unit of the invention. The liquid crystal displayunit 60 of this embodiment comprises a thin-film transistor arraysubstrate 61, a transparent opposed substrate 62 provided in parallelwith, and separately from, the thin-film transistor array substrate 61,and a liquid crystal layer 63 sealed between the thin-film transistorarray substrate 61 and the opposed substrate 62.

As in the conventional structure shown in FIG. 6, a plurality of sourcewiring lines in the longitudinal rows and a plurality of gate wiringlines in the transverse rows are arranged in the above-mentionedthin-film transistor array substrate 61 into a matrix shape, as viewedflat from the upper surface of the opposed substrate 62. A number ofregions surrounded by the source wiring lines and the gate wiring linesserve individually picture element sections. A picture element electrode35 comprising a transparent conductive material such as ITO (indium-tinoxide) is formed in a region corresponding to each picture elementsection, and a thin-film transistor T2 is provided near each pictureelement electrode 35.

FIG. 3 illustrates, in an enlarged form, a part of the thin-filmtransistor T2 provided in a region corresponding to a picture elementsection surrounded by the source wiring lines and the gate wiring linesand the periphery thereof: a number of picture element sections areformed and arranged on the thin-film transistor array substrate 61 toform a display screen for the liquid crystal display unit 60.

The thin-film transistor array substrate 61 of this embodiment comprisesa gate electrode 40 made of a light-shielding conductive material suchas Cr or Mo provided on a substrate 36 in each picture element section;a gate insulating film 41 covering the gate electrode 40 and thesubstrate 36; a semiconductor active film 42 smaller in size than thegate electrode 40, laminated on the gate insulating film 41 on the gateelectrode 40; and ohmic contact films 43 and 44 each comprising an n⁺layer or the like laminated separately from each other with a gap on thecenter portion side of the semiconductor active film 42, at a positionaligned with the ends of the semiconductor active film 42. The gateelectrode 40 is formed into a size larger in width and depth than thesemiconductor active film 42 so as to prevent a light from a back lightprovided under the substrate 36 from entering the semiconductor activefilm 42.

There is provided a low-resistance silicon compound film 64 comprising asilicide film or the like, which covers the upper surface and the leftside surface of the ohmic contact film 43 to the left in FIG. 3 (farside from the picture element electrode 35 in FIG. 3) and the left sidesurface of the semiconductor active film 42 provided thereunder, i.e.,covers the superposed portion of the semiconductor active film 42 andthe ohmic contact film 43. A source electrode 66 comprising a highlyconductive metal material such as Al or an Al alloy is formed so as tocover the low-resistance silicon compound film 64, coated partially onthe gate insulating film 41 adjacent thereto.

There is provided a low-resistance silicon compound film 65 comprising asilicide film or the like, which covers the upper surface and the rightside surface of the ohmic contact film 44 to the right in FIG. 3 (closerside from the picture element electrode 35 in FIG. 3) and the right sidesurface of the semiconductor active film 42 provided thereunder, i.e.,covers the superposed portion of the semiconductor active film 42 andthe ohmic contact film 44. A drain electrode 68 comprising a highlyconductive metal material such as Al or an Al alloy is formed thereon.The silicide film as mentioned above is formed by dispersing atoms of ametal such as Mo, Cr, W, Ta, Pt, Nb, Zr or Ni into an Si film, and amongothers, molybdenum silicide formed by dispersing Mo is particularlypreferable.

Further, a passivation film 69 is provided on these films so as to coverthese films. The picture element electrode 35 is formed on thepassivation film 69 on the right end of the drain electrode 68, and isconnected to the drain electrode 68 via a connecting conductor 71provided in a contact hole 70 formed in the passivation film 69.

A color filter 52 and a common electrode film 53 are laminatedsequentially from the opposed substrate 62 side on the liquid crystalside of the opposite substrate 62 provided to the thin-film arraysubstrate 61. The color filter 52 mainly comprises a black matrix 54 forcovering a portion of the thin-film transistor, a portion of the gatewiring and a portion of the source wiring not contributing to displayand a color picture element section 55 for allowing transmission of alight passing through the portion contributing to display in the pictureelement regions provided with the picture element electrode 35, andfurther for performing color display. The color picture element sections55 are required for the structure of the liquid crystal display unit forcolor display, and provided one for each picture element section. R(red), G (green) and B (blue) color picture element sectionsrepresenting three original colors are arranged regularly or at randomso as to avoid deviation of colors so that adjacent picture elementsections are different in color from each other.

In the sectional structure shown in FIG. 3, orientation films providedon the liquid crystal side of the thin-film transistor array substrate61 and on the liquid crystal side of the opposed substrate 62 areomitted, and polarizing plates provided on the outside of the thin-filmtransistor array substrate 61 and on the outside of the opposedsubstrate 62.

In the structure shown in FIG. 3, the source electrode 66 and the drainelectrode 68 are connected to the semiconductor active film 42 via thelow-resistance silicon compound films 64 and 65 comprising silicidefilms, and further, via the ohmic contact films 43 and 44 on thesemiconductor active film, and the source electrode 66 and the drainelectrode 68 are not in direct contact with the semiconductor activefilm 42. Hall blocking effect is therefore certainly available.Furthermore, since there is no risk of the highly conductive metalmaterial such as Al or an Al alloy composing the source electrode 66 andthe drain electrode 68 being dispersed in the semiconductor active film42, it is possible to reduce OFF-current as a thin-film transistor T2.

When laminating the ohmic contact films 43 and 44 on the semiconductoractive film 42, it is possible to continuously form films without theneed to take out the substrate from the film forming apparatus. It istherefore possible to achieve satisfactory electrical connection on theinterface between the semiconductor active film 42 and the ohmic contactfilms 43 and 44. It is thus possible to obtain a satisfactory value ofON-current for the thin-film transistor T2 of this embodiment.

A typical manufacturing method of the thin-film transistor T2 having thestructure shown in FIG. 3 will now be described with reference to FIG.4.

The process comprises the steps of first preparing a substrate 36 shownin FIG. 4(A); forming a conductive film made of a conductive metalmaterial such as Cr or Mo by means of a film forming apparatus such as asputtering unit on this substrate; then, coating thereon a resist forpattern exposure; removing unnecessary portions through etching; then,applying patterning for peeling off the resist; forming a plurality ofgate wiring lines (not shown), and at the same time, forming a gateelectrode 40 connected to the gate wiring for each positioncorresponding to the picture element section.

The process comprises further steps of, upon forming the gate electrode40, forming a gate insulating film 41 of an insulating material such asSiN_(x) so as to cover the gate wiring and the gate electrode 40 bymeans of a film forming apparatus; and then, continuously forming anisland-shaped semiconductor active film 42 smaller in size than the gateelectrode 40 and an ohmic contact film 43 a at a position opposite tothe gate electrode 40 at the gate insulating film 41 on the gateelectrode 40. In order to form the semiconductor active film 42 and theohmic contact film 43 a, it suffices to continuously laminate filmsserving as materials for these films over the entire surface of the gateinsulating film 41 by means of a film forming apparatus, and then, useportions remaining after removal of unnecessary portions by etching asthe semiconductor active film 42 and the ohmic contact film 43 a.

Then, a silicide preparatory thin film 80 comprising Mo, Ta, W, Cr, t,Nb, Zr or Ni is formed so as to cover the ohmic contact film 43 a andthe gate insulating film 41, and subjected to a heat treatment. As aresult of this heat treatment, silicide films 81 are generated atportions on the both side surfaces of the semiconductor active film 42in contact with the silicide preparatory film 80, and at the both sidesurface portions of the ohmic contact film 43 a in contact with thesilicide preparatory film 80. After the heat treatment, for example, byremoving the low-resistance silicon compound preparatory film (silicidepreparatory film) 80 by means for selectively removing the same by theuse of an etching solution comprising a mixed solution of iodic acid,fluoric acid and acetic acid, the portion of the low-resistance siliconcompound preparatory thin film 80 laminated on the gate insulating film41 is removed, and only the portion forming the silicide film 81 remainsunetched. There is thus available a silicide film 81 wrapping the ohmiccontact film 43 a and the semiconductor active film 42 as shown in FIG.4(C).

Then, a conductive film comprising a conductive metal material such asAl or an Al alloy is formed on the silicide film 81 and the gateinsulating film 41.

Then, by removing the central upper portion of the semiconductor activefilm 42 by the use of the lithographic technology, and removing theohmic contact film 43 a, the silicide film 81 and the conductive film onthe center portion of the semiconductor active film 42 as shown in FIG.4(D), it is possible to form ohmic contact films 43 and 44 on the bothend portions of the semiconductor active film, and to form a sourceelectrode 66 and a drain electrode 68 as coated onto the individualsilicide films. There is thus available a thin-film transistor T2 of thestructure shown in FIG. 4(D).

The source wiring, not shown in the drawing, may be formedsimultaneously with film formation and etching when forming the sourceelectrode 46 on the gate insulating film 41.

When manufacturing a thin-film transistor T2 by the method describedabove, it is possible to continuously form the semiconductor active film42 and the ohmic contact film 43 a formed thereon, and also tocontinuously laminate the both layers in an identical vacuum atmospherewithout breaking the vacuum atmosphere in the film forming apparatus. Itis thus possible to connect and form in a satisfactory state ofelectrical connection the semiconductor active film 42 and the ohmiccontact film 43 a formed thereon without causing impurities to be mixedin the connection interface between the both films. In the structureshown in FIG. 4(D), the source electrode 66 has no part in contact withthe semiconductor active film 42, and the drain electrode 68 has no partin contact with the semiconductor active film 42. It is thereforepossible to reduce OFF-current as a thin-film transistor T2.

As a result, there is available a thin-film transistor substrate havinga thin-film transistor T2 excellent in transistor characteristics with ahigh ON-current and a low OFF-current.

EXAMPLES Example 1

A rectangular gate electrode as shown in FIG. 1 having a width of 13 μmwas formed with a Cr film on a transparent glass substrate, and the thusformed gate electrode was covered with an SiN_(x) gate insulating film.A semiconductor film comprising a rectangular a-Si having a width of 9μm was formed on the gate insulating film on the center portion of thegate electrode. On the both ends thereof, ohmic contact film having awidth of 3 μm comprising phosphorus-doped n⁺ a-Si layers were formed,respectively. Further, a low-resistance silicon compound film comprisinga phosphorus (P)-doped n⁺ a-Si layer was laminated so as to cover theohmic contact films, and a source electrode and a drain electrodecomprising Al were thereon laminated, thereby manufacturing a thin-filmtransistor having the sectional structure shown in FIG. 1.

The results of measurement of OFF-current (I_(OFF)) and ON-current(I_(ON)) of the resultant thin-film transistor are shown in FIG. 5.

Comparison of the OFF-current and the ON-current represented by thecurves shown in FIG. 5 and the OFF-current and the ON-current for theconventional structure shown in FIG. 10 reveals that, by adopting thestructure shown in FIG. 1, it is possible to reduce OFF-current andincrease ON-current.

Example 2

A rectangular gate electrode as shown in FIG. 3 having a width of 13 μmwas formed with a Cr film on a transparent glass substrate, and the thusformed gate electrode was covered with an SiN_(x) gate insulating film.A semiconductor film comprising a rectangular a-Si having a width of 9μm was formed on the gate insulating film on the center portion of thegate electrode. On the both ends thereof, ohmic contact films having awidth of 3 μm comprising phosphorus-doped n⁺ a-Si layers were formed,respectively. Further, a low-resistance silicon compound film comprisingan Mo silicide film was formed so as to cover the upper and sidesurfaces of the ohmic contact films and the side surface of thesemiconductor active film thereunder. A source electrode and a drainelectrode comprising Al were laminated thereon, thereby manufacturing athin-film transistor having the sectional structure as shown in FIG. 3.

The silicide film was formed by laminating the semiconductor active filmand the ohmic contact films, then, forming the Mo low-resistance siliconcompound preparatory thin film (silicide preparatory film) covering thesame, performing elementary dispersion through a heat treatment forheating to 280° C., removing by etching the silicide film on the centerportion of the semiconductor active film, and removing by etching theohmic contact films thereunder.

The results of measurement of OFF-current (I_(OFF)) and ON-current(I_(ON)) of the resultant thin-film transistor agreed with the resultsof Example 1 shown in FIG. 5.

Comparison of the OFF-current and the ON-current represented by thecurves shown in FIG. 5 and the OFF-current and the ON-current for theconventional structure shown in FIG. 10 reveals that, by adopting thestructure shown in FIG. 3, it is possible to reduce OFF-current andincrease ON-current.

According to the thin-film transistor of the present invention, asdescribed above, the material for the source electrode or the drainelectrode is never brought into direct contact with the semiconductoractive film, and the source electrode and the drain electrode can beconnected to the semiconductor active film via the ohmic contact filmsand the low-resistance silicon compound film. It is thus possible toreduce OFF-current of the transistor.

By adopting the structure in which the ohmic contact films are laminatedonly on the side upper surfaces of the semiconductor active film, it ispossible to fabricate the semiconductor active film and the ohmiccontact films into necessary shapes through patterning after laminatingthese films continuously, and to continuously form films withoutbreaking the vacuum atmosphere upon film formation. Impurities aretherefore prevented from mixing in the interfaces between the laminatedsemiconductor active film and ohmic contact films, thus ensuringsufficient electrical connection of these films, and enabling toincrease sufficiently ON-current.

As a result, there is provided a thin-film transistor substratesatisfying requirements of both a high ON-current and a low OFF-currentso far unavailable in the conventional thin-film transistors.

The present invention further provides a liquid crystal display unithaving a thin-film transistor substrate satisfying the characteristicsof a high ON-current and a low OFF-current.

According to the method of the invention, furthermore, direct contactbetween the source and drain electrodes and the semiconductor activefilm can be avoided, and the source electrode and the drain electrodecan be connected to the semiconductor active film via the ohmic contactfilms and the low-resistance silicon compound film, thus making itpossible to provide a thin-film transistor capable of reducingOFF-current.

When laminating the ohmic contact films only on the upper surface of thesemiconductor active film, it is possible to fabricate the semiconductoractive film and the ohmic contact films by patterning into necessaryshapes after laminating these films continuously, thus making itpossible to provide a thin-film transistor permitting achievement of asufficient electrical connection of the semiconductor active film andthe ohmic contact films and a high ON-current.

According to the method of the invention, furthermore, continuousformation of the semiconductor active film and the ohmic contact filmson the gate insulating film permits lamination of these films in acleaned state of the interfaces between these films, thus enabling toachieve satisfactory electrical connection between these films.

What is claimed is:
 1. A thin-film transistor substrate comprising: agate electrode comprising a light shading conductive material providedon a transparent substrate; a gate insulating film provided on saidsubstrate to cover said gate electrode; a semiconductor active filmprovided oppositely via said gate insulating film above said gateelectrode, a width and a depth of the gate electrode being larger thanthat of the semiconductor active film thereby shielding the entiresemiconductor active film from light impinging on the semiconductoractive film from a bottom of the substrate, the semiconductor activefilm being planar on the gate electrode; a pair of ohmic contact filmsseparately provided on an upper surface of said semiconductor activefilm; a low-resistance silicon compound film comprising a materialselected from a-Si:n⁺ and a silicide, the low-resistance siliconcompound film being disposed over a surface and one side of both of theohmic contact films, both sides of the semiconductor active film, and asurface of the gate insulating film to cover the corresponding ohmiccontact film and the side of the semiconductor active film superposed onthe corresponding ohmic contact film; and a source electrode and a drainelectrode provided on said low-resistance silicon compound film to blockholes from the semiconductor active film, the source and drainelectrodes comprising one of aluminum and an aluminum alloy.
 2. A liquidcrystal apparatus, wherein the thin-film transistor substrate accordingto claim 1 serves as one of a pair of substrates having a liquid crystallayer in between.
 3. A thin-film transistor substrate according to claim1, wherein the semiconductor active film comprises a-Si.
 4. A thin-filmtransistor substrate according to claim 1, wherein the silicidecomprises a compound formed by diffusing an element selected from thegroup consisting of Mo, Cr, W, Ta, Pt, Nb, Zr, and Ni, into a Si film.5. A thin-film transistor substrate according to claim 1, wherein thegate electrode comprises one of Cr and Mo.
 6. A thin-film transistorsubstrate according to claim 1, wherein the ohmic contact film comprisesn⁺ a-Si doped with phosphorus.
 7. A thin-film transistor substrateaccording to claim 2, wherein the semiconductor active film comprisesa-Si.
 8. A thin-film transistor substrate according to claim 2, whereinthe silicide comprises a compound formed by diffusing an elementselected from the group consisting of Mo, Cr, W, Ta, Pt, Nb, Zr, and Ni,into a Si film.
 9. A thin-film transistor substrate according to claim2, wherein the gate electrode comprises one of Cr and Mo.
 10. Athin-film transistor substrate according to claim 2, wherein the ohmiccontact film comprises n⁺ a-Si doped with phosphorus.